Amplifier linearized by negative feedback



H. BENDEL AMPLIFIER LINEARIZED BY NEGATIVE FEEDBACK June 4, 1940.

Fild Dec. 4, less INVENTOR HERMANN BENDEL Kg ATTORNEY Patented June 4,I940 UNITED STATES AMPLIFIER LINEARIZED B Y NEGATIVE FEEDBACK HermannBendel, Berlin, Germany, assignor to Siemens & Halske,Aktienge'sellschaft, Siemenstaclt, near Berlin, Germany a corporation ofGermany Application December 4, 1936,1SerialNo. 114,175

, In Germany December 12, 1935 4Clzlill'lS. (01.179-171) ance Rk.Contradistinct from what is "the case It is a well-known fact thatnon-linear distortions of an amplifier circuit arrangement may bediminished by means of negative feedback or degeneration. Thesenon-linear distortions are reducedso much more markedly, the greatersuch degeneration may be made, that is to say, the more marked thereduction of the gain or amplification which is accomplishable, in thepresence of stable conditions, by degeneration. However, a limitation issoon imposed upon degeneration by undesired phase shifts occasioned bycoupling. means. And in the presence of certain frequencies, spuriouscouplings arise which will causethe amplifier to oscillate. r

In the accompanying drawing, Fig. 1 illustrates in schematic form anamplifier stage incorporating a usual form of negative feedback; Fig. 2illustrates a form of the. invention wherein the shield is connected tothe junction point of the output transformer and the feedbackresistance;

. Figs. 1a and 2a are simplified diagrams of the arrangements shown inFigs. 1 and 2 respectively;

and, Fig.3 illustrates in diagrammatic form an embodiment of theinvention in a condenser resistance coupling circuit.

An essential part so far as the phase shifts are concerned, played,inter alia, by the capacitances setup between the coupling means andother parts of the amplifier arrangement. Fig.

1 shows an amplifierfstage comprising negative feedback of the usualkind. The load impedance B is associated by way of a transformer withthe platecircuit of the tube. Theinput circuit E is connected betweenthe grid and the junction point of the primary winding of the outputtransformer and the feedback resistance Rk. The shield S of thetransformer is grounded in the usual way. This circuit scheme allowsonly comparatively little. degeneration. In one particular. case of atwo-stage amplifier cascade, natural (spontaneous) oscillations werefound to arise upon a reduction of the amplification of only 0.5

neper. l e

e Starting upon the idea that the smallness of permissible degenerationis ascribable onlyto the shift of phase caused by the capacity C,certain circuit means and methods are suggested in this inventionwhereby essential capacities between the coupling impedance and theparts of the feedback impedance turned away from the said iminv thecircuit scheme Fig. 1, this precludes the arising of capacitancesbetween the plate impedance and the end of the feedback resistanceturned away therefrom. 'In Fig. 1, the capacitances represented by C-are elfective between the coupling impedance and the grounded screen S.These capacitances' necessarily are also provided between the end of thefeedback impedance which is away from the coupling impedance for thereason thatthis end is connected through ground to the screen.

Figs. 1a and 2a illustrate the corresponding equivalent circuit diagramswhich make the effect of the capacitances upon the phase shift clear.Fig. 1a shows immediately that between the inner resistance of theamplifier tube and the feedback resistance there is inserted aquadripole (four-polenetwork) which, as is well known,.occasions aconsiderable phase shift between the potentials arising across the innerresistance of the tube and the feedbackresistance. The said shil'tconsists of the phase shifts of the various elements so that, even inthe presence of slight shifts of the constituentelements, the sum totalof the shift may amount to 211-. In the circuit scheme Fig. 2a, atwo-pole system is interposed between the innerresistance of the E. M. Fand the feedback resistance for which, of course, the phase shift maynotbe over a 1 7 The resistance of thistwo-pole system as a general rule islow in contrast with the coupling resistance R1; andthe inner resistanceof the tube (particularly where screen-grid tubes are used); hence, evenif the two-pole network were to un-.

dergo a maximum phase shift, thecurrent in the coupling resistance andthus also the feedback voltagewould alter but inessentially.-

The idea underlying this invention, as will be understood, is notconfined to the output transformers of the amplifier tubes, but mayanalogously be used also in connection with input transformers. If coilrather than transformer coupling is involved, naturally the same conch--tions arise as regards the shift of the phases, though it may here becombated by the identical ways and means.- i

However, the value of permissible degeneration or negative feedback maybe raised not only for purely inductive coupling means, but also in thecase of condenser resistance coupling. A case of this kind is shown inFig. 3. I The plate circuit of the amplifier tube contains a seriesarrangement comprising the coupling resistance Ba. and the feedbackresistance Rk. Inasmuch as the capacitances between the plate resistanceand the base (creasing) especially Where ultra-high frequencies areamplified, play an essential part, it follows that basically the samephase shift should be anticipated as with a circuit organizationaccording to Figs. 1 or 1a. In other words, the same phase shifts shouldbe expected in the arrangement shown in Fig. 3 as are found in thearrangement shown in Fig. 1 for the reason that in Fig. 3 also thecapacitances between the coupling resistance Ra and the'base' plate,that is,

ground, become very important. To be sure, the grounded end of thefeedback resistance R1; is

in this case so connected to the base plate or ground that thecapacitances between the casing and the coupling resistance also existbetween the grounded end of the feedback'resistance and the couplingresistance. What must also be taken into consideration in thisconnection is that for extremely'high frequencies the inductivecomponents of the so-called ohm coupling resistances should and couldnot. be disregarded. Hence, in such a case, according to this invention,a shield S is provided between the plate resistance and the grounded endof thev feedback resistance with a view to precluding capacitivecouplings, the said shield being connected with the joint or junctionpoint of Ra. and Rk.

- If a so-called voltage feedback rather than a current feedbackasshownin Fig. 2--is concerned in which the feedback potential is taken offacross the terminals of a resistance connected in series with the loadresistance, there apply fundamentshield which must be associated withpoint A.

.To'what particular point should in such an instance be united theshield possibl provided for the load resistance connected in parallelrelation to the voltage divider will depend upon the requirements of agiven case.

p In a transformer-coupledscreen-grid amplifier which had been designedfor handling from to 20,000 cps'., it has been feasible by using theinvention to raise the admissible degeneration from 0.5' to 4 neper.

WhatI claim is:

1. In an amplifier circuit, an electronic tube provided with an anode,'a cathode and a grid electrode, an input circuit for said tubeincludinga pair-of input terminals one of said terminals being connectedto the grid electrode, an

output circuit for said tube comprising aconnection between the cathodeand ground and a connection between the anode and ground, said lastnamed connection including a'coupling coil and a feedback resistance inseries whereby one end of the feedback resistance is grounded, a

connection between the other of said input terminals and the un-groundedend of the feedback resistance, a utilizing circuit, means including acoil coup-led to the first mentioned coil for coupling the utilizingcircuit to the output circuit of saidtube, and a shielding meansinterposed betweensaid two windings, said shielding means beingconnected to the ungrounded end of said feedback resistance.

2. In a negative feedback amplifier, an electronic tube provided with ananode, a cathode and a grid electrode, an output circuit. for saidamplifier comprising a connection between cathode and ground and aconnection between the anode and ground, said last named connectionincluding a plate impedance device and a feedback resistor in serieswhereby one end of the feedback resistor is grounded, an input circuitfor said electronic tube including a pair of input terminals, one ofsaid terminals being connected to the grid electrode, the other of saidinput terminals being connected tc -the ungrounded end of the feedbackresistance, a utilizing circuit connected effectively across the plateimpedance and means for shielding the plate impedance, said shieldingmeans being connected to the ungrounded end of saidfeedback resistor.

3. Ali-near amplifier'comprising, an electronic tube having an-anode, acathode and a grid electrode, an output circuit for said tube comprisinga shielded plate impedance and a feedback resistor connected in seriesin the order named between the anode and cathode of the tube, a shieldfor the plate impedance, an input circuit for said tube including a pairof input terminals, one of said terminals beingconnected to the gridelectrode, means for connecting the other of said terminals to saidshield and to a point which is intermediate the plate impedance and thefeedback resistor, means for connecting the cathode end of said resistorto ground, a utilizing circuit including a pair of input terminals,means including a coupling capacity for connecting one of the last namedinput terminals to the anode of the tube and means for connecting theother of said last two named input terminals to said intermediate pointof the output circuit,

4. In amplifying'apparatus an electronic 'tube having an'anode, acathode and a grid electrode, an'output c'ircuitforsaidtube comprisingan output impedance and a feedback resistor connected in'series intheorder named between the anode and cathode of tlfe tube, means 'forgrounding the cathode of the tube whereby 'the end of the feedbackresistor remote from the output impedance is grounded, an input circuitfor said tube including a pair of input terminals, one of said terminalsbeing connected to the grid electrode of thetube, the "other inputterminal being connected tothe ungrounded end of the feedback resistor,a utilizing circuit effectively connected across theoutput impedance andmeans for preventing coupling between the output im-' pe'dance and thegrounded end of the feedback resistance through'any inherent capacityeffects therebetween comprising a shielding means for said outputimpedance, said'shielding means being conductively connected totheungrounded end of the feedback resistor.

HERMANN BENDEL.

